Second harmonic generation (SHG) has been developed largely as an important technique which causes visible light radiation using a solid laser. A pair of mirrors which generates a resonating beam is installed in a laser cavity in order to increase the optical effectiveness of a second harmonic generation. Second harmonic generation is a second-order non-linear optical technique and its effectiveness is in proportion to the square of the output of a pumping laser. The development of a laser diode matched to the absorption band of a solid laser material such as Nd:YAG enables the minimization of a solid laser. However, a diode-pump solid laser still does not have an enough output to produce an effective second harmonic generation.
U.S. Pat. Nos. 3,858,056 and 5,093,832 each proposed a technique for achieving an effective second harmonic generation with a relatively low output, through an intra-cavity method in which an intra laser resonator is provided.
The techniques disclosed in the above patents maximize the Q value of a laser resonator, that is, the ratio (Q=Dp/Dd) of a parallel light density (Dp) to a diffusion light density (Dd), is maximized, by which the SHG output of an accumulated resonating beam in a resonator is increased. Therefore, this SHG effectiveness of an intra-cavity depends on how much the loss of the intra-resonator can be minimized. The basic element of a small intra-cavity SHG laser which has thus far been proposed, including the above two patents, comprises two high-reflection mirrors which provide a high Q cavity, a gain medium (e.g., YAG) which is arranged between the above two mirrors, a non-linear bifringent crystalline element (e.g., KTP), a Brewster plate, etc.
Referring to U.S. Pat. No. 3,858,056, as shown in FIG. 2, harmonics 22 output from a laser cavity 10 in which mirrors 12 and 13, a dynamic laser rod, e.g., YAG 19, and a non-linear bifringent crystalline element, e.g., KTP 18 are prepared, is partially split using a beam splitter 21, and then the split part of harmonics passes through a filter 24. Then, beam strength thereof is measured using a photo-detector 23, applying an alternate time-sequential method. Then, the temperature of a non-linear bifringent crystalline element is controlled by a difference between the strength values sequentially obtained from a multivibrator 27 and a bidirectional counter 28. However, the overall system structure for this method of SHG is complicated, and controlling the temperature of a non-linear bifringent crystalline element using only a beam strength error signal is difficult, which causes problems in putting this method to practical use.
U.S. Pat. No. 5,093,832 describes a resonating beam and discloses more particulary a feedback method of SHG by measuring a beam angle. As shown in FIG. 1, this patent describes a structure in which an optical resonator 10 having mirrors 12 and 13 at both ends and an optical path of a reflection beam reflected from mirror 13 is different from a processing path of an incident beam emitted from a laser diode 9. Part of the output beam is isolated through a beam splitter 54, and then is photo-detected by a photo-detector 23 to be fed-back to a peltier element 80 using a temperature controller 33. However, in this technique, a feedback circuit includes photo-detector 23, a differential amplifier 66, a temperature controller 33, and a thermistor 86, which are separately installed outside an optical resonator 10. Particulary, it is difficult to minimize an SHG module because a number of independant elements are arranged inside an optical resonator.